The objective of these experiments is to elucidate the rules governing the normal development of the motor innervation of rat skeletal muscles. Muscle fibers are initially polyneuronally innervated by multiple axons. With maturation, all but one of the inputs per fiber are eliminated. The major mechanism of elimination is retraction. We are characterizing the retraction process using serial-section electron microscopy, horseradish peroxidase tracer experiments and immunocytochemical techniques. Competition for synaptic sites is a general phenomenon during the development of central and peripheral nervous tissues; it is crucial to understand what factors determine whether a particular synaptic terminal survives or involutes. The morphological and cytochemical properties of these two classes of terminals will be compared to ascertain what influences the apparent process of selection. Labile and stable terminals will be identified both by peroxidase uptake into stimulated terminals and acid phosphatase cytochemistry. Reinnervation of injured nervous tissue and the return of function is disappointing in humans. A better understanding of the normal process of synapse formation is likely to suggest means of improving the repair of injured and diseased nervous tissues.